Combination pills work in two ways. First, they prevent your body from ovulating. This means that your ovaries won’t release an egg each month. Second, these pills cause your body to thicken your cervical mucus. This mucus is fluid around your cervix that helps sperm travel to your uterus so it can fertilize an egg. The thickened mucus helps prevent sperm from reaching the uterus.
Progestin-only pills also work in a few different ways. Mainly, they work by thickening your cervical mucus and by thinning your endometrium. Your endometrium is the lining of your uterus where an egg implants after it’s fertilized. If this lining is thinner, it’s harder for an egg to implant in it, which will prevent a pregnancy from growing. In addition, progestin-only pills may prevent ovulation.
How Do I Use Birth Control Pills?
Combination pills come in a variety of formats. These include monthly packs, which follow either 21-day, 24-day, or 28-day cycles. Extended regimens can follow 91-day cycles. For all of these formats, you take one pill each day at the same time of day.
Progestin-only pills, on the other hand, only come in packs of 28. As with combination pills, you take one pill at the same time every day.
How Effective Are Birth Control Pills?
If taken correctly, birth control pills are very effective in preventing pregnancy. According to the CDC, both the combination pill and the progestin-only pill have 9 percentTrusted Source failure rates with typical use. That means out of 100 women using the pill, 9 would get pregnant.
To be fully effective, progestin pills must be taken within the same three-hour time period every day.
There is slightly more flexibility with combination pills. In general, you should try to take combination pills at the same time each day, but you can take them within the same daily 12-hour window and still have pregnancy protection.
Certain medications may make either type of pill less effective. These include:
- rifampin (an antibiotic)
- certain HIV medications such as lopinavir and saquinavir
- certain antiseizure medications such as carbamazepine and topiramate
- St. John’s wort
The pill may also be less effective if you have diarrhea or vomiting. If you’ve had a stomach illness, check with your doctor to see if you’re at risk of pregnancy. Use a backup method of contraception until you know it’s safe not to do so.